Apparatus and method for perforating a subterranean formation

ABSTRACT

Method and apparatus are presented for perforating a subterranean formation so as to establish fluid communication between the formation and a wellbore, the wellbore having casing cemented therein, the casing having a cement sheath therearound. The casing is perforated with a mechanical perforator and thereafter a propellant material is ignited within the casing thereby perforating the cement sheath. The formation may thereafter be stimulated with an acid stimulator. The mechanical perforator may include use of a toothed wheel, or a needle-punch perforator. The propellant may be deployed in a sleeve and may comprise an abrasive material.

TECHNICAL FIELD

[0001] This invention relates to new and improved methods of perforatinga cemented well bore casing and the surrounding cement.

BACKGROUND OF THE INVENTION

[0002] In the process of establishing an oil or gas well, the well istypically provided with an arrangement for selectively establishingfluid communication with certain zones in the formation traversed by thewell. A typical method of controlling the zones with which the well isin fluid communication is by running well casing into the well and thensealing the annulus between the exterior of the casing and the walls ofthe wellbore with cement. Often the casing is expanded once it is run-into the well. Thereafter, the well casing and cement may be perforatedusing mechanical or chemical means at preselected locations by aperforating device or the like to establish a plurality of fluid flowpaths between the pipe and the product bearing zones in the formation.

[0003] Much effort has been devoted to developing apparatus and methodsof perforation. Explosive charges are sometimes used to constructperforating guns, such as disclosed in U.S. Pat. No. 5,701,964 to Walkeret al. Attempts have been made to increase the effectiveness ofexplosive perforation methods by combining them with propellant fracturedevices. An example of such attempts is disclosed in U.S. Pat. No.5,775,426 to Snider, et al, wherein a sheath of propellant material ispositioned to substantially encircle at least one shaped charge. Underthis method, the propellant generates high-pressure gasses, which cleanthe perforations left by explosive charges.

[0004] Problems exist with the use of explosives to perforate casing,however. Unfortunately, the process of perforating through the casingand then though the layer of cement dissipates a substantial portion ofthe energy from the explosive perforating device and the formationreceives only a minor portion of the perforating energy.

[0005] Further, explosives create high-energy plasma that can penetratethe wall of the adjacent casing, cement sheath outside the casing, andthe surrounding formation rock to provide a flow path for formationfluids. Unfortunately, the act of creating the perforation tunnel mayalso create some significant debris and due to the force of theexpanding plasma jet, drive some of the debris into the surrounding rockthereby plugging the newly created flow tunnel. Techniques have beendeveloped to reduce the effect of the embedded debris, such asperforming the perforation operation in an under-balanced condition orperforming backflushing operations following perforation.

[0006] Perforating in an under-balanced condition causes the formationfluids to surge into the wellbore yielding a cleaning effect. Afterperforating in an under-balanced condition the well must be “killed” bycirculating out the produced fluids and replacing them with heaviercompletion fluids. Oftentimes significant amounts of completion fluidare then lost to the formation, which can be expensive and potentiallydamaging to productivity. Fluid loss may result in formation damage dueto swelling of formation clay minerals, particle invasion into theformation, dissolution of matrix cementation thereby promoting finesmigration, and by interaction between the completion fluids and theformation fluids causing emulsion or water blocks or changes in thewetability of the formation sand. Fluid loss pills may also be required,which can be expensive and damaging.

[0007] Mechanical perforation may avoid many of these problems. Devicesfor mechanically perforating a well casing without the use of explosivesare also known in the art and, in fact, predate the use of explosives.Laterally movable punches are exemplified by the devices shown in theJobe U.S. Pat. No. 2,482,913, Frogge, U.S. Pat. No. 3,212,580, Grable,U.S. Pat. No. 3,720,262, and Gardner, U.S. Pat. No. 4,165,784, which areeach incorporated herein by reference. Toothed wheel perforators areexemplified by the devices showing in Graham, U.S. Pat. No. 1,162,601;Noble, U.S. Pat. No. 1,247,140; Baash, U.S. Pat. No. 1,259,340; Baash,U.S. Pat. No. 1,272,597; Layne, U.S. Pat. No. 1,497,919; Layne, U.S.Pat. No. 1,500,829; Layne, U.S. Pat. No. 1,532,592; Jerome, U.S. Pat.No. 4,106,561; and Hank, U.S. Pat. No. 4,220,201, which are eachincorporated herein by reference.

[0008] It is also known in the art to run into a well a liner that ispre-perforated with the openings filled by shearable plugs. Such adevice is exemplified by U.S. Pat. No. 4,498,543 to Pye, which isincorporated herein by reference.

[0009] Unfortunately, these mechanical and shearable plug methods ofperforation are of limited use where the casing is cemented in place andthese methods do not perforate the fluid bearing formation.

SUMMARY OF THE INVENTION

[0010] Method and apparatus are presented for perforating a subterraneanformation so as to establish fluid communication between the formationand a wellbore, the wellbore having casing cemented therein, the casinghaving a cement sheath therearound. The casing is perforated with amechanical perforator and thereafter a propellant material is ignitedwithin the casing thereby perforating the cement sheath. The formationmay thereafter be stimulated with an acid stimulator. The mechanicalperforator may include use of a toothed wheel, or a needle-punchperforator. The propellant may be deployed in a sleeve and may comprisean abrasive material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The accompanying drawings are incorporated into and form a partof the specification to illustrate several examples of the presentinventions. These drawings together with the description serve toexplain the principals of the inventions. The drawings are only for thepurpose of illustrating preferred and alternative examples of how theinventions can be made and used and are not to be construed as limitingthe inventions to only the illustrated and described examples. Thevarious advantages and features of the present inventions will beapparent from a consideration of the drawings in which:

[0012]FIG. 1 is an elevational cross-sectional view of a downholeportion of a cased and cemented well;

[0013]FIG. 2 is an elevational cross-sectional view of a mechanicalperforator as described herein;

[0014]FIG. 3 is an elevational cross-sectional view of amultiple-wheeled mechanical perforator as described herein;

[0015]FIG. 4 is an elevational cross-sectional view of a needle-punchperforator as described herein;

[0016]FIGS. 5A and 5B are elevational cross-sectional views of aperforation method described herein;

[0017]FIG. 6A is an elevational cross-sectional view of a perforationmethod described herein;

[0018]FIG. 6B is a detail of said a step of method;

[0019]FIG. 6C is an elevational cross-sectional view of a perforationmethod described herein;

[0020]FIG. 6D is a detail of an embodiment which maybe employed in saidmethod;

[0021]FIG. 6E is a detail of an embodiment which may be employed in themethod;

[0022]FIG. 6F is a detail of an embodiment which may be employed in themethod;

[0023]FIG. 6G is a detail of an embodiment which may be employed in themethod;

[0024]FIG. 6H is a detail of an embodiment which may be employed in themethod;

[0025]FIG. 7A is a cross-sectional view of a propellant deployed inperforated casing;

[0026]FIG. 7B is a top-view cross-section of a propellant and abrasiveparticulate deployment system;

[0027]FIG. 7C is a top-view cross-section of the system of FIG. 7Bduring deployment;

[0028]FIG. 7D is an elevational cross-sectional detail of FIG. 6C;

[0029]FIG. 7E is an elevational cross-sectional representation of aperforated and acid washed formation.

DETAILED DESCRIPTION

[0030] The present inventions are described by reference to drawingsshowing one or more examples of how the inventions can be made and used.In these drawings, reference characters are used throughout the severalviews to indicate like or corresponding parts. In the description whichfollows, like or corresponding parts are marked throughout thespecification and drawings with the same reference numerals,respectively. The drawings are not necessarily to scale and theproportions of certain parts have been exaggerated to better illustratedetails and features of the invention. In the following description, theterms “upper,” “upward,” “lower,” “below,” “downhole,” “longitudinally,”and the like, as used herein, shall mean in relation to the bottom, orfurthest extent of, the surrounding wellbore even though the wellbore orportions of it may be deviated or horizontal. Correspondingly, the“transverse” or “radial” orientation shall mean the orientationperpendicular to the longitudinal orientation. In the discussion whichfollows, generally cylindrical well, pipe and tube components areassumed unless expressed otherwise.

[0031]FIG. 1 shows a portion of hydrocarbon well 10. Wellbore 12 extendsthrough formation 14 having at least one producing, or hydrocarbonbearing, zone 16. To avoid communication with non-producing zones,wellbore 12 are typically cased and cemented and thereafter perforatedalong the producing zones. Wellbore 12 is lined with casing 18 andcement 20. Methods of cementing and casing are well known in the art. Itis understood that the casing may be traditional or expandable casing.In the illustrated wellbore 12, a work string 24 has been run in,including tool subassembly 26, which may house mechanical, chemical orexplosive perforators, or other well tools.

[0032] Mechanical Perforators:

[0033] Devices for mechanically perforating a well casing predate theuse of explosives. Toothed wheel perforators are exemplified by thedevices shown in U.S. Pat. No. 1,162,601 to Graham, U.S. Pat. No.1,247,140 to Noble, U.S. Pat. No. 1,259,340 to Baash, U.S. Pat. No.1,272,597 to Baash, U.S. Pat. No. 1,497,919 to Layne, U.S. Pat. No.1,500,829 to Layne, U.S. Pat. No. 1,532,592 to Layne, U.S. Pat. No.4,106,561 to Jermone, and U.S. Pat. No. 4,220,201 to Hank, each of whichare incorporated herein in their entirety by reference for all purposes.

[0034] Referring to FIG. 2, a retractable-toothed perforator wheel 100is fixed to the lower end of a work string 24 that has been lowered intothe cased wellbore 12. The perforator is positioned within the casing 18at the depth of the producing zone 16 of the formation 14.

[0035] The perforator 100 includes a main body 102, a wheel arm 104, anda cutter wheel 106 with a plurality of cutting teeth 108.

[0036] The cutter wheel 106 may be of any size to fit within the casing18 and plurality of circumferentially spaced, generally radially cutterteeth 108 may be extendable, that is movable between a home position110, as illustrated in FIG. 2, and a cutting position 112. The teeth108, if extendable, are moveable via an appropriate actuating device 118such as spring mountings, lever arms, piston assemblies or the like.Appropriate locking mechanisms may be necessary to maintain the teeth inthe cutting position.

[0037] The wheel arm 104 pivots or otherwise moves, if necessary, toallow the cutting wheel to be moved between a run-in position 114 and anoperable position 116, as illustrated in FIG. 2. The wheel arm 104 canbe moved between the run-in position 114 and the operable position 116by use of an arm actuator and may be spring-mounted, hydraulically orair driven, electrically actuated or by any other means known.

[0038] In operation, the perforator 100 is lowered into the wellbore 12with the wheel arm in the run-in position 114 such that the cutter doesnot contact the casing 18. The teeth 108, if extendable, are preferablyin the home position 110 during run-in operations with all of the teeth108 spaced inwardly from the casing. The exterior of the wheel 106 issimilarly spaced away from the casing. The perforator 100 is lowered toa desired depth adjacent the production zone 16 where the teeth 108 areextended to the cutting position 112. The wheel arm 104 is then movedsuch that the wheel 106 is brought into contact with the casing 18.Preferably, the entire perforator is then pulled uphole by raising thework string 24. It is understood that the cutter tool can be operated ina top-down method. The cutter wheel 106 is forced to rotate, driving theteeth 108 into and through the casing 18. The entire perforator 100 israised the desired distance along the production zone 16 to provide aline of perforations along this length. Once the desired length ofperforations is completed, the cutter wheel 106 and arm 104 are returnedto their run-in positions. The perforator can then be rotated and movedwithin the casing and one or more addition lines of perforation made, asdesired.

[0039] One of the drawbacks of mechanical perforation is the time andexpense involved in making the multiple trips up and down the casingneeded to perforate an adequate number of rows of holes in the casingwall. This is especially true where perforation is desired over alengthy vertical interval of the wellbore. FIG. 3 shows an arrangementof multiple cutter wheels 106 configured on a single perforator tool100. The multiple wheels 106 are arranged to produce multiple rows ofperforations 124 along the casing wall 18. FIG. 3 shows three separatecutting wheels 106, but it is understood that greater or fewer wheelscan be used as desired. The multiple wheels may employ pivot arms,retractable teeth, and various actuators and locking mechanisms andother mechanisms as are known in the art as needed.

[0040]FIG. 4 shows a needle-punch perforator 140 having a plurality ofmovable needles 142 supported on a perforator body 144. The needles aremovably mounted to the perforator and extend in a generally radialdirection. The needle-punch perforator 140 is run-in to the casing 18 toa desired depth with the needles 142 in a retracted position 148 suchthat the needles do not interfere with movement of the tool 140. Theneedles are preferably directed radially outward when in the run-in, orretracted, position, as shown, but can be mounted to point in anydirection so as not to interfere with the run-in procedure. Once theperforator 140 is positioned within the production zone 16, the needles142 are moved to an extended position 150 wherein the needles 142perforate the casing wall 18. Extension of the needles 142 isaccomplished via an actuating means 152. FIG. 4 shows a substantiallyconical expansion plug 154 which, when pulled through the perforatorbody 144, forces the plurality of needles 142 outward and through thecasing 18. The needles 142 can slide through holes in the perforatorbody 144, as shown, or the perforator body 144 itself, or moveable partsthereof, may expand carrying the needles 142 thereon.

[0041] After perforation of the casing, the needles can be retractedfrom the casing and withdrawn, along with the perforator, from thewellbore. Alternately, the needles can be sheared or otherwise brokenoff from the perforator and left in place in the casing wall. In such acase, the needles can then be dissolved in an acid solution injectedinto the wellbore.

[0042] The perforator tools shown in the various figures may be usedseparately or in conjunction with one another or other well tools. Itmay be desirable to combine the perforator run-in with the run-in forother well tools. The complexity of the system may outweigh theadvantages of combining multiple operations in a single trip, however,all of the methods of perforation described herein may be performed ineither a bottom-up or top-down method. The perforators may be used inwellbores which have been cemented or are not cemented or withtraditional or expandable casing. In the case of cemented casing, themechanical perforators may have teeth which perforate into or throughthe cemented portion surrounding the casing. More typically, the teethwill perforate the casing wall but not through the entire thickness ofthe cement sheath. Other methods may be used to perforate through thecement and, if desired, to fracture the formation itself, as describedherein.

[0043] Pre-Perforated Casing:

[0044] Among the many types of downhole well completions is one in whicha pre-perforated liner, screen or casing is positioned adjacent theproduction zone. The pre-perforated liner may be left sittingunsupported in the open hole, or the annular space between the wellboreand the outside of the pre-perforated liner can be filled with apermeable material, such as a gravel pack, or the space may be filledwith cement which must later be perforated. Pre-perforated liners can beespecially useful where the wellbore sidewall material is poorlyconsolidated or contains or is composed of shale, clays, silicates andthe like and the produced or injected fluids contain or are composed ofwater.

[0045] Difficulties have been experienced in running pre-perforatedliners into wells, especially wells penetrating reservoirs containinghigh-pressure fluids, more particularly high temperature geothermalfluids and most particularly dry geothermal steam wells. When attemptshave been made to run a pre-perforated liner into such wells, the highpressure formation fluids quickly pass through the perforations and upthe liner to the surface where they escape, resulting in considerabledanger to the workmen running the liner.

[0046] It has been the practice in the past to first inject into thewell a fluid, in sufficient volume to provide hydrostatic head tocounterbalance the formation pressure and “kill” the well. Theperforated liner can then be safely run into the well and the injectedwater subsequently removed. However, this manner of killing the well hasnot been satisfactory since the reason for running the liner in thefirst place is that the wellbore may contain shale or similar unstablematerials. These materials can swell and collapse into the open hole assoon as contacted by the injected water. Thus, the wellbore becomesrestricted with detritus and the liner cannot be lowered into place.

[0047] In certain well operations, such as in cementing casing, it isknown to run into a well pre-perforated liner whose openings have beenfilled with plugs, and to later run a cutting tool down the liner toremove the plugs and open the openings in the liner. Such a method isdescribed in U.S. Pat. No. 4,498,543 to Pye, which is incorporatedherein by reference.

[0048] It is also known in the art to run into a wellbore pre-perforatedbase pipe having a protective shell over a well screen, the shell havingopenings which have been filled with a sacrificial material, forexample, zinc, aluminum and magnesium. The sacrificial plugs temporarilyprevent dirty completion fluid from passing through the pre-perforatedscreen shell as it is run in to the wellbore, thereby protecting thescreen from plugging. After the screen assembly is in place downhole,the shell plugs are dissolved by an acid or other corrosive solution,for example, hydrogen chloride (HCL) or hydrogen fluoride (HF), or by acaustic solution such as sodium hydroxide (NaOH) or potassium hydroxide(KOH). The specific acid or caustic solution used is determined in partby the characteristics of the well. After dissolution of the plugs,further well operations can be carried out. Such a system is describedin U.S. Pat. No. 5,355,956 to Restarick and is incorporated herein byreference.

[0049] It has become common to insert expandable casing into wellbores.The casing, in its smaller diameter pre-expanded state, is run into thewellbore to a desired depth. The casing is then expanded, usually bypulling a specially designed expansion plug through the casing, to alarger diameter expanded state. If it is desired to cement theexpandable casing in place, cement is placed in the annular spacebetween the casing and the wellbore. Typically the cement is placedwhere desired in a slurry, or “wet” form, and the casing is thenexpanded prior the cement drying or “setting.” This helps ensure thatthe annular cavity is properly filled with cement. Unfortunately, theshearable and dissolvable plugs tend to tear, break or pull away fromthe casing during the expansion process.

[0050]FIGS. 5A and 5B show a pre-perforated assembly 200 having a casing18 which has pre-formed holes or perforations 202 in the wall thereof.The casing 18 is expandable and is run-in to the wellbore 12 in anunexpanded state 204, as seen in FIG. 5A, then expanded, by means knownin the art, to an expanded state 206, as seen in FIG. 5B. Cement 20 isplaced into the space 208 between the wellbore wall and the exterior ofthe casing 18, typically prior to expansion of the casing. The casing 18is typically expanded before the cement 20 has hardened or “set.” Theperforations 202 are temporarily sealed by sacrificial plugs 210. In oneembodiment, each plug 210 is fabricated from a sacrificial metal such aszinc, aluminum and magnesium, which may be dissolved when contacted by ahigh pH acid or a low pH base solution. It is desirable that the metalselected be characterized by a relatively faster rate of etching ordissolution when contacted by an acid or base solution, as compared tothe rate that the casing 18 is affected.

[0051] The plugs 210 can be threadingly engaged, friction fit orotherwise secured with casing perforations 202. During initial assembly,each perforation 202 is sealed by engagement of the plugs 210. Thethickness of the plug 210 is selected so that it will be completelydissolved within a predetermined period of exposure to a corrosive, acidsolution or base solution, for example, for four hours. As the plugs 210dissolve, the perforations 202 are opened up to permit the flow offormation fluid through the casing 18. In this embodiment, the plugs 210may be hollow, having a relief pocket 212 therein, or may be solid. Ifused with expandable casing, the plugs 210 must be robust to expand withthe casing without breaking. Examples of suitable materials include:aluminum, brass, bronze, and fiberglass reinforced epoxy resin.

[0052] Additionally, the plugs can be made of rubber, plastic or othermaterial which is solid at low temperatures but melts or dissolves overtime when exposed to higher temperatures.

[0053] In another embodiment, the perforations 202 are temporarilysealed by plugs 210 which are shearable. A shearable plug 214 is shownin FIGS. 5A and 5B. Although dissolvable and shearable plugs can be usedsimultaneously, this would be highly unusual. Shearable plug 214 has abody portion 216 intersected by a relief pocket 212, which is sealed, bya stub portion 218. The relief pocket 212 extends partially into stubportion 218. The stub portion 218 projects radically into the bore 220of the casing 18. Once the casing 18 is in place, the perforations 202are opened mechanically by shearing the shearable plugs 214. This isperformed with a milling tool, which is run on a concentric tubingstring. The stub portion 218 is milled, thereby opening relief pocket212. Alternatively, the plugs are removed by flooding the bore of thescreen mandrel 18 with an acid solution, so that the plugs aredissolved. In that arrangement, the plugs are constructed of a metal,which dissolves readily when contacted by an acid solution, for example,zinc, aluminum and magnesium. Zinc is the preferred metal since itexhibits the fastest dissolving rate. Where the plugs 214 are to besheared, the plugs can be made of any solid material. Particularlysuitable are materials which are capable of withstanding considerablefluid pressure differential yet can be rather easily cut or broken.Examples of suitable materials include steel, cast iron, aluminumalloys, brass and plastics.

[0054] Plugs 210 preferably have a wellbore protrusion 222 whichprojects radially outward from casing 18 into the wellbore area. Suchprotrusions 222 may be used with plugs of dissolvable design 210 orshearable design 214. The protrusions 222 can be sized to contact thewellbore surface, as shown in FIG. 5B. If protrusions 222 are utilizedon expandable casing, the plugs 210 must be of a robust material capableof expansion and appropriately sized to expand with the casing 18.Examples of suitable materials include: steel, cast iron, aluminumalloys, brass and plastics.

[0055] In another embodiment, the plugs 210 are reactive plugs 224, asshown in FIGS. 5A and 5B. Again, it would be unlikely to simultaneouslyemploy soluble plugs 210, shearable plugs 214 and/or reactive plugs 224,but all are included in FIGS. 5A and 5B for ease of reference. Reactiveplugs 224 can employ protrusion 222, as can the other types of plugs.

[0056] Each reactive plug 224 can be mounted in a pre-formed recess 226in the casing 18 or otherwise connected to the casing. As the casing 18is expanded, the reactive plugs 224 expand as well. In the presence of apre-selected additive 228, which can be introduced downholeindependently or as part of the cement slurry, the reactive plugs 224expand to many times their original size and in a prescribed geometricpattern. The expanded reactive plugs 224 would thereby createperforation tunnels into and/or through the cement 20.

[0057] After the reactive plugs 224 have expanded and the cement 20 hasset, the reactive plugs 224 can be dissolved in a suitable fluid.

[0058] The reactive plugs 224 can be made of any suitable material whichwill expand in the presence of an additive, as is known in the art. Forexample, the plugs 224 can be made of an elastomer, such as EPDM(Ethylene Propylene) which swells in the presence of diesel. Appropriateplug material, additives, and solvents can be selected as wellconditions demand.

[0059] FIGS. 6A-6H show a pre-perforated casing 18 having extendableperforation “fingers” 300, or darts, mounted thereon. The fingers 300are attached to the outside of casing 18 in a run-in position 306, asseen in FIG. 6A. Pre-formed perforations 302 are temporarily pluggedwith plugs 304. Once the perforated casing is in place in the wellbore,the fingers 300 are moved to an extended position 308, as seen in FIG.6B. Cement 20 is placed into the wellbore 12 and the casing 18 isexpanded prior to the cement setting. As the casing 18 is expanded, thefingers 300 contact the wellbore 12 and are forced radially inward,thereby piercing the temporary plugs 304, and moving to a final position316 as seen in FIG. 6C.

[0060] The fingers 300 can be hinged, tagged or otherwise attached tothe casing 18 at attachment means 310. The fingers 300 are movablebetween the run-in position 306 and the extended position 308. Movementbetween the positions 306 and 308 may be achieved by any means known inthe art. For example, the drill tool string bearing the perforatedcasing can be rotated creating a centrifugal force, which rotates thefingers from the run-in to the extended position. As another example,the darts 300 may have a wire 312, as shown in FIG. 6D, extendingradially outward from the dart 300 and also extending uphole. The wire312 contacts the wellbore 12. As the perforation tool is run-in to thewellbore 12 the wire 312 simply drags along the wellbore wall, bendingas necessary so as not to affect the run-in procedure. Once the tool hasreached the desired depth in the wellbore 12, the tool is pulled upholea short distance, where the wire 312 contacts the wellbore wall “bites”into the wall. The casing 18 is moved uphole, but the wire 312 maintainsits position in the wellbore, thereby forcing the dart 300 to rotatedownward into an extended position 308, seen in FIG. 6E. The sameprocedure can be used with a textured surface on the exterior of thedart, where the texturing allows free downhole movement but “bites” uponuphole movement of the tool string.

[0061] An alternative embodiment employing a spring device 314 is shownin FIGS. 6F-6H. FIG. 6F employees a torsion spring device 313 capable ofrotating the dart 300. FIGS. 6G 6H illustrate use of a coil springdevice 315 rotating the dart 300 between a run-in position 306 (FIG. 6G)and an extended position 308 (FIG. 6H). Other methods of moving darts300 between run-in and extended positions will be readily apparent tothose skilled in the art.

[0062] Temporary plugs 304 may be pierced when the fingers 300 arerotated to the extended position 308 or when the fingers 300 are forcedradially inward to a final position 316 by contact with the wellbore.Temporary plugs may be made of aluminum, brass, bronze, and fiberglassreinforced epoxy resin.

[0063] Propellants:

[0064] Following the perforation methods described herein, the casing 18has perforations extending through the walls thereof. In some instances,for example, as shown in FIG. 5B, the perforations extend into thecement sheath 20 and perhaps extend to the wellbore wall 12. Where theperforations do not extend through the cement sheath, it is necessary tofracture the cement sheath and in any case it is necessary to fracturethe formation. In a sand control environment, it may be desirable toplace holes in the casing but not through the cement sheath so that thecement acts as a fluid loss control device during subsequent activity.

[0065] Fracturing may be accomplished several ways. Propellant 400 isdeployed downhole adjacent perforations 202. As seen in FIG. 7A, thepropellant 400 can be deployed as part of the completion in “stick” or“sleeve” form. The propellant 400 is then ignited in a manner similar tothe tubing conveyed perforating methods which are known in the art. Thepropellant 400 can also be deployed via wireline after completionequipment is in place or by any other method known in the art.

[0066] Upon ignition, the propellant 400 will vacate the casing 18through perforations 202, thereby cleaning the perforations, andfracture the cement sheath 20 and the formation zone 16.

[0067] The propellant 400 can also be deployed in combination with anabrasive particulate 402, as shown in FIG. 7B, and as known in the art.Including erosive or abrasive particulate 402 with the high-energy fluidstream of the ignited propellant 400 enhances scouring of the cementsheath 20 and formation 16. At the time of detonation, and in somecases, for a few seconds thereafter, the particulate matter 402 isexpelled into the formation as seen in FIG. 7C. The particulate 402abrades and penetrates the cement sheath and the formation, therebycreating flow connectivity.

[0068] Another method of perforation is possible in the perforationmethod shown in FIG. 6C, or in any perforation application employingextendable fingers or darts. The fingers 300 can include an explosivecharge for perforating formation zone 16, as seen in FIG. 7D. The finger300 has a barrel portion 320 which extends radially from casing 18 intocement sheath 20 and preferably to formation zone 16. Barrel 320 housesan explosive perforating device 322 which may include initiators,detonators and charges as in known in the art. Once the fingers 300 aredeployed in the extended position 308, the perforating device 322 isignited and perforates zone 16.

[0069] Alternately, the extended fingers 300 can act as nozzles,directing the ignited propellant from a propellant sleeve deployed inthe casing. When the propellant is ignited it penetrates the tips 324 ofthe fingers 300 and fractures the formation zone 16 as shown in FIG. 7E.

[0070] Acid Stimulation:

[0071] It may be desirable, after perforation and ignition of thepropellant, to stimulate the formation by displacing an acid 404 intothe formation 16 to enhance flow connectivity as shown in FIG. 8. Use ofacid stimulation to enhance connectivity is known in the art, and anytype of acid stimulation and method of deployment known in the art maybeemployed.

[0072] Having thus described our invention, it will be understood thatsuch description has been given by way of illustration and example andnot by way of limitation, reference for the latter purpose being had tothe appended claims.

What is claimed is:
 1. A method of perforating a subterranean formationwhich is penetrated by a wellbore, the wellbore having casing cementedtherein, a cement sheath around the casing, so as to establish fluidcommunication between the formation and the wellbore, the methodcomprising the steps of: perforating the casing using a mechanicalperforator; and thereafter igniting a propellant material disposed inthe perforated casing thereby perforating the cement sheath.
 2. A methodas in 1 further comprising the step of stimulating the formation with anacid stimulator.
 3. A method as in 1 wherein the step of perforating thecasing using a mechanical perforator further includes perforating atleast some distance into the cement sheath.
 4. A method as in 1 whereinthe mechanical perforator comprises at least one toothed wheel.
 5. Amethod as in 4 wherein the at least one toothed wheel includedextendable teeth.
 6. A method as in 1 wherein the mechanical perforatorcomprises needle-punch perforator.
 7. A method as in 1 wherein thepropellant material comprises a propellant stick.
 8. A method as in 1wherein the propellant material comprises a propellant sleeve.
 9. Amethod as in 1 wherein the step of igniting the propellant materialfurther comprises expelling an abrasive material through theperforations in the casing thereby scouring the perforations in thecement sheath.
 10. A method as in 1 wherein the propellant further actsin part to perforate the formation.
 11. A method as in 9 wherein theabrasive material acts in part to perforate the formation.
 12. A methodas in 1 further comprising the step of deploying in the casing aperforator subassembly including the mechanical perforator.
 13. A methodas in 12 wherein the mechanical perforator includes at least one toothedwheel.
 14. A method as in 1 further comprising the step of deploying inthe casing a propellant subassembly including the propellant material.15. A method as in 14 wherein the propellant subassembly furthercomprises an abrasive material.
 16. A method as in 15 wherein the stepof igniting the propellant material further comprises expelling theabrasive material through the perforations in the casing.
 17. A methodas in 2 further comprising the step of deploying in the casing an acidstimulation subassembly for delivery of the acid stimulator to theformation.
 18. A method as in 1 wherein the casing is expandable casing.19. An apparatus for perforating a subterranean formation which ispenetrated by a wellbore, so as to establish fluid communication betweenthe formation and the wellbore, the wellbore having casing cementedtherein, a cement sheath around the casing, the apparatus comprising: amechanical perforator subassembly for creating perforations at least inthe casing; and a propellant subassembly for creating perforations in atleast the cement sheath.
 20. An apparatus as in 19 further comprising anacid stimulation subassembly for delivery of the acid stimulator to theformation.
 21. An apparatus as in 19 wherein the mechanical perforatorcapable of perforating at least some distance into the cement sheath.22. An apparatus as in 19 wherein the mechanical perforator subassemblycomprises at least one toothed wheel.
 23. An apparatus as in 22 whereinthe at least one toothed wheel includes extendable teeth.
 24. Anapparatus as in 19 wherein the mechanical perforator subassemblycomprises a needle-punch perforator.
 25. An apparatus as in 19 whereinthe propellant subassembly comprises a propellant stick.
 26. Anapparatus as in 19 wherein the propellant subassembly comprises apropellant sleeve.
 27. An apparatus as in 19 wherein the propellantsubassembly comprises propellant and an abrasive material for expulsionthrough the perforations in the casing created by the mechanicalperforation assembly.
 28. An apparatus as in 19 wherein the propellantsubassembly is further capable of creating perforations in theformation.
 29. An apparatus as in 27 wherein the abrasive material iscapable of perforating the formation.
 30. An apparatus as in 19 whereinthe casing is expandable casing.
 31. A method of perforating asubterranean formation which is penetrated by a wellbore, so as toestablish fluid communication between the formation and the wellbore,the method comprising the steps of: cementing casing in the wellborethereby creating a cement sheath around at least a portion of thecasings perforating the casing using a mechanical perforator; andthereafter igniting a propellant material disposed in the perforatedcasing.
 32. A method as in 31 wherein the step of cementing casingfurther comprises expanding the casing.
 33. A method as in 31 furthercomprising the step of stimulating the formation with an acidstimulator.
 34. Method as in 31 wherein the step of perforating thecasing using a mechanical perforator further includes perforating atleast some distance into the cement sheath.
 35. A method as in 31wherein the mechanical perforator comprises at least one toothed wheel.36. A method as in 31 wherein the mechanical perforator comprises aneedle-punch perforator.
 37. A method as in 31 wherein the propellantmaterial comprises a propellant sleeve.
 38. A method as in 3, whereinthe step of igniting the propellant material further comprises expellingan abrasive material through the perforations in the casing.
 39. Amethod as in 32 further comprising the step of stimulating the formationwith an acid stimulator.
 40. A method as in 32 wherein the step ofperforating the casing using a mechanical perforator includesperforating at least some distance into the cement sheath.
 41. A casingperforator apparatus for perforating casing disposed in a wellbore, theapparatus comprising: a perforator body; and a plurality of toothedwheels movably mounted to the perforator body.
 42. An apparatus as in 41the casing perforator having three toothed wheels, each wheel having adifferent axis of rotation.
 43. An apparatus as in 41 wherein at leastone of the toothed wheels has extendable teeth.
 44. An apparatus as in41 further comprising means for moving the toothed wheels into contactwith the casing.
 45. An apparatus as in 41 wherein the casing iscemented in the wellbore, having a cement sheath around the casing. 46.An apparatus as in 45 wherein the plurality of toothed wheels have teethcapable of perforating at least some distance into the cement sheath.47. A casing perforator apparatus for perforating casing disposed in awellbore, the apparatus comprising: a perforator body; and at least onetoothed wheel movably mounted to the body, each wheel having a pluralityof extendable teeth movable between a retracted position and an extendedposition.
 48. An apparatus as in 47 the at least one toothed wheelcomprising three toothed wheels.
 49. An apparatus as in 47, each toothedwheel having an actuator for moving the teeth to the extended position.50. An apparatus as in 49, each toothed wheel having a locking mechanismfor at least temporarily locking the teeth in the extended position . ..
 51. A casing perforator apparatus for perforating casing disposed in awellbore, the apparatus comprising: a perforator body; and a pluralityof perforator needles movable between a retracted position and anextended position; and an actuating means for moving the needles fromthe retracted position to the extended position.
 52. An apparatus as in51 the actuating means capable of moving the needles from the extendedposition to the retracted position.
 53. An apparatus as in 51 whereinthe needles are shearable from the perforator body.
 54. An apparatus asin 51 wherein the needles are mounted in a generally radial positionwhen in the retracted position.
 55. An apparatus as in 53 wherein theneedles are soluble in acid solution.
 56. An apparatus as in 51 whereinthe actuating means is a substantially conical expansion plug
 57. Anapparatus as in 51 wherein the casing is cemented in the wellbore acement sheath around the casing.
 58. An apparatus as in 57 wherein theneedles are capable of perforating through the casing and at least somedistance into the cement sheath.
 59. A method of perforating a casing ina wellbore, the method comprising: positioning a perforator in thecasing, the perforator having a plurality of perforator needles movablemounted thereon, the needles in a retracted position; and moving theneedles to an extended position and perforating the casing with theneedles.
 60. A method as in 59 further comprising the step of moving theneedles from the extended position to the retracted position.
 61. Amethod as in 59 further comprising the steps of disconnecting theneedles from the perforator.
 62. A method as in 61 further comprisingdissolving the needles.
 63. A method as in 59 wherein the step of movingthe needles includes moving an extension plug through the perforator.64. A method as in 59 wherein the casing is cemented in the wellbore, acement sheath around the casing, and further comprising the step ofperforating at least some distance into the cement sheath.
 65. A wellcasing apparatus for a subterranean formation which is penetrated by awellbore, the casing comprising: a substantially tubular casing having acasing wall with a plurality of perforations therethrough; and aplurality of sacrificial plugs secured to the casing wall and sealingthe plurality of perforations.
 66. An apparatus as in 65 wherein thecasing and plugs are expandable, such that the plugs remain secured tothe casing wall, sealing the plurality of perforations, when the casingis expanded.
 67. An apparatus as in 65 wherein the sacrificial plugs aresoluble in an acid or caustic solution.
 68. An apparatus as in 67wherein the plugs comprise aluminum.
 69. An apparatus as in 66 whereinthe sacrificial plugs are soluble in an acid or caustic solution.
 70. Anapparatus as in 69 wherein the plugs comprise aluminum.
 71. An apparatusas in 66 wherein the sacrificial plugs are shearable.
 72. An apparatusas in 71, the casing wall enclosing a casing bore, and wherein each plughas a body portion engaging the casing wall and having a stub portionprotecting into the casing bore, the body portion intersected by arelief pocket.
 73. As in 65 wherein the sacrificial plugs furthercomprise a wellbore protrusion projecting into the wellbore.
 74. Anapparatus as in 66 wherein the sacrificial plugs further comprise awellbore protrusion projecting into the wellbore.
 75. An apparatus as in74 wherein the plug protrusions comprise EPDM.
 76. An apparatus as in 65wherein the sacrificial plugs comprise reactive plugs.
 77. An apparatusas in 14 wherein the reactive plugs are mounted to the casing wall inpreformed recesses therein.
 78. An apparatus as in 76 wherein thereactive plugs comprise an elastomer.
 79. An apparatus as in 81 whereinthe reactive plugs expand in a prescribed geometric pattern in thepresence of a pre-selected additive.
 80. An apparatus as in 79 whereinthe reactive plugs expand in the presence of diesel.
 81. An apparatus asin 66 wherein the sacrificial plugs comprise reactive plugs.
 82. Anapparatus as in 81 wherein the reactive plugs are mounted to the casingwall in preformed recesses.
 83. An apparatus as in 81 wherein thereactive plugs are mounted to the casing wall in preformed recesses. 84.An apparatus as in 81 wherein the reactive plugs expand in a prescribedgeometric pattern in the presence of a pre-selected additive.
 85. Anapparatus as in 84 wherein the reactive plugs expand in the presence ofdiesel.
 86. An apparatus as in 76 wherein the reactive plugs dissolve inan acid or caustic solution.
 87. An apparatus as in 81 wherein thereactive plugs dissolve in an acid or caustic solution.
 88. A method ofcompleting a well having a wellbore penetrating a subterraneanformation, the method comprising the steps of: placing a substantiallytubular casing having a casing wall enclosing a casing bore, the casingwall having a plurality of sacrificial plugs secured to the casing walland sealing the plurality of perforations; and rupturing the sacrificialplugs, thereby establishing fluid communication between the wellbore andthe casing bore.
 89. A method as in 88 further comprising the step ofexpanding the casing and sacrificial plugs such that the plugs remainsecured to the casing wall and sealing the plurality of perforationsduring expansion of the casing and plugs.
 90. A method as in 89 furthercomprising the step of cementing the casing in the wellbore.
 91. Amethod as in 88 wherein the step of rupturing the plugs furthercomprises dissolving the plugs.
 92. A method as in 91 wherein the plugsare dissolved in an acid solution.
 93. A method as in 91 wherein theplugs comprise aluminum.
 94. A method as in 89 wherein the step ofrupturing the plugs further comprises dissolving the plugs.
 95. A methodas in 89 wherein the step of rupturing the plugs comprises shearing aportion of the plugs.
 96. A method as in 95 wherein the plugs eachcomprise a body portion secured to the casing wall and stab portionprojecting in to the casing bore, the body portion intersected by arelief pocket.
 97. A method as in 95 wherein the plugs each comprise aprotrusion extending into the wellbore.
 98. A method as in 90 the stepof cementing creating a cemented sheath around the casing, and whereinthe plugs comprise protrusions projecting into the wellbore and into thecement sheath.
 99. A method as in 90 wherein the plugs are reactiveplugs and further comprising the step of expanding the reactive plugssuch that a protruding portion of each of the plugs projects into thewellbore and into the cement.
 100. A method as in 99 wherein thereactive plugs expand in the presence of a preselected additive.
 101. Amethod as in 100 wherein the step of cementing further comprises thestep of placing the additive into the wellbore adjacent the plugs in thecasing.
 102. A method as in 101 wherein the reactive plugs are anelastomer and the additive is diesel.
 103. A method as in 99 furthercomprising the step of dissolving the reactive plugs after the step ofexpanding the reactive plugs.
 104. A method as in 88 further comprisingthe step of cementing the casing in the wellbore.
 105. A method as in104 wherein the plugs are reactive plugs and further comprising the stepof expanding the reactive plugs such that a protruding portion of eachof the plugs projects into the wellbore and into the cement.
 106. Amethod as in 105 wherein the reactive plugs expand in the presence of apreselected additive.
 107. A method as in 105 wherein the step ofcementing further comprises the step of placing the additive into thewellbore adjacent the plugs in the casing.
 108. A method as in 107wherein the reactive plugs are an elastomer and the additive is diesel.109. A method as in 105 wherein the plugs are reactive plugs and furthercomprising the step of expanding the reactive plugs such that aprotruding portion of each of the plugs projects into the wellbore andinto the cement.
 110. An apparatus for completing a well in asubterranean formation penetrated by a wellbore, the apparatuscomprising: a casing having a casing wall; a plurality of perforationsthrough the casing wall; a plurality of plugs corresponding to theplurality of perforations, the plugs sealing the plurality ofperforations; and a plurality of extendable fingers secured to thecasing wall adjacent the plurality of the perforations, each of thefingers movable between a run-in position wherein the fingers do notinterfere with the casing being run-in to the wellbore, and an extendedposition wherein the fingers project radially from the casing wall. 111.An apparatus as in 110 wherein the casing is expandable.
 112. Anapparatus as in 111 wherein each of the fingers is movable between theextended position and a final position wherein each finger pierces acorresponding plug.
 113. An apparatus as in 111 wherein each fingercomprises an explosive charge for perforating the subterraneanformation.
 114. An apparatus as in 110, the casing wall enclosing acasing bore, and further comprising a propellant subassembly in thecasing bore ignitable to vacate the casing bore through the plurality ofperforations.
 115. The apparatus as in 110, wherein each finger ispivotally attached to the casing wall.
 116. The apparatus as in 110,wherein a wire extends from each finger, the wire for engaging thewellbore and moving the finger between the run-in and the extendedpositions.
 117. The apparatus as in 110, the fingers movable between therun-in and extended positions by a spring device.
 118. The apparatus asin 117, wherein the spring device is a torsion spring device.
 119. Amethod of perforating a subterranean formation which is penetrated by awellbore, so as to establish fluid communication between the formationand the wellbore, the method comprising the steps of: running a casinginto the wellbore, the casing having a casing wall, a plurality ofperforations through the casing wall, a plurality of plugs sealing theplurality of perforations, and a plurality of fingers secured to thecasing wall adjacent the plurality of perforations, the fingers in arun-in position wherein the fingers do not interfere with running thecasing into the wellbore; moving each of the plurality of fingers to anextended position wherein each finger projects radially outward from thecasing wall; and thereafter igniting a propellant, the propellantexiting through the plurality of perforations and the plurality offingers thereby perforating the formation.
 120. A method as in 119further comprising the step of expanding the casing.
 121. method as in119 wherein the propellant is mounted in the plurality of fingers. 122.A method as in 119 wherein the propellant is disposed in the casing.123. A method as in 122 further comprising the step of running apropellant subassembly into the casing.
 124. A method as in 119 furthercomprising the step of cementing the casing in the wellbore.
 125. Amethod as in 124 further comprising the steps of expanding the casing.126. A method as in 119 further comprising the step of moving each ofthe plurality of fingers from the extended position to a final positionwherein each of the fingers pierces a corresponding plug.
 127. A methodas in 126, the step of moving the fingers to a final position furthercomprising expanding the casing such that the fingers contact thewellbore wall.
 128. A method as in 119 wherein each finger is pivotallyattached to the casing wall.
 129. A method as in 119 wherein a wireextends from each finger, the wire for engaging the wellbore and movingthe finger between the run-in and the extended positions.
 130. A methodas in 119 the fingers movable between the run-in and extended positionsby a spring device.
 131. A method as in 130 wherein the spring device isa torsion spring device.